Posts about: "Fuel (All)" [Posts: 345 Pages: 18]

A flight test (at least one - it's often duplicated) is performed as a basic part of aircraft/engine certification. One engine with all boost pumps off and on 'suction' feed - the other engine with normal aircraft boost pump operation (for what should be obvious reasons). Start, taxi, takeoff, and climb in that configuration until the test engine quits due to fuel starvation as a result of the engine fuel pump cavitation (done using "unweathered" fuel - once fuel has been at altitude for a period of time (hours or more - i.e. 'weathered'), most of the dissolved gases in the fuel have vented off, and suction feed works far better - often up to cruise altitudes).
I don't think this test is ever done during normal operations or maintenance (at least not on purpose) as it is very abusive to the engine driven fuel pump - the sort of cavitation that this causes rapidly erodes the pumping surfaces (it's SOP to replace the engine driven fuel pump after such a test).

I hate to disappoint you, but the people (like me) who design, test, and certify aircraft are not idiots. We design for failures. Yes, on rare occasion, something gets missed (e.g. MCAS), but we know that aircraft power systems sometimes fail (or suffer short term interuptions) and we design for that. EVERY VALVE IN THE FUEL SYSTEM MUST BE POWERED TO CHANGE STATE!!!! If electrical power is lost, they just stay where they are. The engine fuel valve must be powered open, and it must be powered closed. Same with the spar valve. The pilot moves a switch, that provides electrical signals to the spar valve and the engine fuel valve to open or close. It's not complicated and has been in use for decades.
TCMA (not TCAM) - Thrust Control Malfunction Accommodation - is a FADEC based system. It's resident in the engine FADEC (aka EEC) - the ONLY inputs from the aircraft that go into the TCMA is air/ground (to enable) and thrust lever position (to determine if the engine is doing what it's being commanded to do. The FADEC has the ability to shutdown the engine via the N2 overspeed protection system - this is separate from the aircraft run/cutoff signal, although it uses the same HPSOV to effect the shutdown. That same system is used by TCMA to shutoff fuel if it determines the engine is 'running away'.

Hint, you might try going back a few pages and reading where all this has been posted previously.

I don't want to believe it either. It defies belief but given certain confirmation bias (sim), panic and inexperience by the FO, it's not beyond the realms of possibility. It explains both the RAT and loss of lift. The alternative right now is a catastrophic electrical failure that wiped out the engines or engine control leading to loss of thrust, or a dual engine failure due to birds, fuel contamination or FOD. They are also valid, though IMO less likely.

This is a constant-pressure hydraulic system, not a little hydraulic ram on a logsplitter. While I assume there are some overpressure relief valves, they're not relevant here.

It uses a variable displacement pump to maintain 5000PSI constant pressure. The swashplate angle is varied to adjust pump output flow: more devices consuming fluid, more flow to keep the pressure up. If the pumps cannot deliver enough fluid, the swashplate reaches the full flow position and the output pressure decreases until flow consumed equals flow produced. Very much like a constant-current constant-voltage power supply.

Running in that area of maximum flow is 100% expected under some conditions, especially if an engine or EDP fails and the electric demand pump is supplying a whole hydraulic system sized for the larger EDP (although I think this would be less of an issue on the 787 as the L/R systems don't do much, but the same variable-displacement pump design has been around for a LONG time including on the 737).

And again, there's a VFD between the aircraft electrical bus and the pump motor, because the pump is 400Hz and the aircraft is wild-frequency. VFDs are very very good at isolating faults unless you are actually looking at a sustained overload on one of four generators .


Virtually every bus will have a feed and one or more cross-ties or back-feeds. A failed contactor is 100% designed for and with possibly the sole exception of RAT-only flight, entirely designed around. Plus, of course, flight on batteries only or PMGs.

No bus is essential on a modern aircraft.

Boeing treats everything electric as a black box but the A380 has this beautifully overkill drawing - given both have 4x generators, 2x APU generators, and a RAT, it should not be entirely dissimilar levels of redundancy:

Note that the reason for some links having two contactors in series (e.g. BTC5/6 or BTC7) is because this is spread across two separate units, so that a fire and total loss of one leaves ~half the aircraft powered and totally flyable.


As per TDR, built into the FADEC logic.

Power-open power-close is very common in commercial/situations where you don't want to be wasting energy 24/7 and don't have a defined position for the valve/damper in case of power loss. Done a bunch of them in ductwork and electrically operated windows - your car likely has them, for example.

In that case, I would think that it is not beyond the realm of remote possibility that for whatever reason there might be at least some of these in the field that will not actually function in the suction mode. And if we are talking about simultaneous dual-flameouts then we're already in the "realm of remote possibility", so they should be looking at these unlikely causes. If they're never tested, it's simply an unknown. Discussions so far just assume that this feature works. From what you say it would not be simple to test all of the in-service engines since the test itself is destructive. Perhaps there is some way to test without grinding up the pumps.

The engine driven fuel pumps are regularly removed and overhauled - usually when the engines go through overhaul (somewhere in the 10-20,000 hour range). The results of these overhauls are monitored, and if there is evidence of unusual deterioration, etc., that will be reflected in the recommended maintenance/overhaul intervals (BTW, this is SOP for virtually every system on the aircraft, regardless of Boeing, Airbus, etc.).
The portion of the engine driven fuel pump that is subject to wear is the high-pressure gear pump - and excessive deterioration will become apparent in the inability to reach max TO thrust. The centrifugal pump (that part responsible for the suction feed) is relatively lightly loaded and seldom experiences excessive wear or deterioration - even when exposed to severe suction feed events.
As I've posted several times, in this business you 'never say never' - but the chances that both engines fuel pumps were deteriorated to the point where they could not adequately provide suction feed fuel to keep the engines running is very, very remote.

I guess it all depends on what you mean!

If the fuel supplies were cut off, causing the engines to stop, is that engine failure ? I'd say not, nothing wrong with the engines until they impacted the buildings etc.

No evidence of RAT deployment - but you're specifically restricting "the evidence" to a blurry amateur video. That alone is not great evidence, but why does that video exist at all? When they lift the relevant section of fuselage, RAT deployment or not is going to be fairly apparent. And Circumstantial evidence is still evidence, no?

No evidence of electrical failure? Do you know that from the downloaded Flight Data?

I would be less suspicious of the high-pressure pumps themselves and more suspicious of the check/bypass valves that are meant to allow fuel to flow straight from the tank into high-pressure fuel pump if the electric pumps fail.

Of course, those should be trivial to bench test.

How about TCMA shuts down both engines for some unknown reason, then AC power fails and RAT deploys. I agree is very low probability but these very low probability events have happened in the past. I just posted AA191 as one example...

A thrust reduction is not an engine failure. Engine shutdown due to an action of crew (or inaction) is not a failure.

There is no evidence of an electrical failure. What evidence? A surviving passenger thought he saw flickering lights? Give me a break.

The word evidence in English has a very specific meaning.

Look for the simplest explanation here and then ask why the worldwide B787 fleet is still flying with no urgent inspection requirements from Boeing or GE. Think about that "evidence".

Hi tdracer, and thanks for your comments.

I hope I never suggested you guys are idiots! I very much doubt that indeed. You cannot be idiots. Planes fly, very reliably. That's evidence enough.

Maybe my analysis is simplistic, but for someone who knows as little about the nuts and bolts that are your profession, I think I'm not doing too badly.

I believe I have made a number of worthy contributions to this thread. Maybe I'm deluded. Too bad. Fact is, over the history of modern aviation, there have been a number of serious design stuff ups that "shouldn't have happened". As far as I'm concerned, the crash of AF447 is bloody good evidence of not considering a very simple, fundamental failure, and should NEVER have happened. The thing is, that would have been sooo easy to avoid. So please, don't get on too high a horse over this.

Thanks for your information about all the fuel control valves. That's cool. Yes, my cars have numerous such systems, from the radiator grilles backward.

And you misunderstand what I meant about "complicates things". Was that deliberate? What I meant was it complicates understanding how a major electrical failure could cause the Fuel Cutoff valves to close, that's all. The valves don't close if unpowered, but if the control is via the FADEC, then what could have caused them to close?

Your explanation of how the Fuel Valves are controlled is rather simplistic too. "The pilot moves a switch, that provides electrical signals to the spar valve and the engine fuel valve to open or close." Seriously? Am I an idiot then? Is it a single pole, single throw switch? Is the valve driven by a stepper motor, or what? A DC Motor and worm drive? Does it have an integral controller? How does the valve drive know when to stop at end of travel? Would you mind elaborating, please?

If the photo of the flaps deployed at the accident site is actually F1 not F5 or if the flaps were pushed out during impact, then this is certainly plausible. I will look for the photo but it's in the thread somewhere. Others are stating they see a gap between the wing and the flap as an argument for the flaps deployed at F5. This was after the decent started..

However, I think their reaction would likely be to apply more power. I know mine would be. But anything is possible!

What happens if you forget to turn on fuel pumps during preliminary prep?

I'm not sure if the engines would start , but once they were running, suction/gravity feed into the engine-driven high-pressure fuel pumps would keep the engine running, just like in an electrical failure.

Expect all the alarms, too.

The 320 starts even if you forget to turn on fuel pumps. Don't ask how I know.

In reply to you, I am solely stating that it's a thrust-side problem. I think I somewhat misinterpreted your post as it looks like you might have been saying that anyway.

In general, I think it's looking like dual engine failure/shutdown cutting electrics. I agree that why it occurred is very unclear. Outside chance of total electrical failure causing dual engine failure rather than the other way around, but that would perhaps be even more concerning a design failure.

Similar to Jeju, we also have what is looking increasingly like a loss of ADS-B data at the moment things went wrong, not just a loss of coverage.

That gives:

• Sound of RAT
• Visible RAT
• (edit: APU door open implies APU autostart)
• Loss of ADS-B out
• Near-total loss of thrust.

The alternate theories seem to be a) flaps (basically discounted), b) suction feed failed after total electrical failure, or c):

• A/T rolled engines back
• Crew interpreted this as dual engine failure
• Crew didn't push throttles forward
• Crew did switch each engine off & on again and maybe deployed RAT manually as well.

I think it has been suggested that the upload only happens every 30 minutes or so.

Loss of electrical power will not affect fuel supply to the engines, if the fuel boost pumps in each tank are inop, suction will open their respective bypass valves. It would take two completely separate electrical commands, to two completely separate LPSOV\x94s to drive them to the closed position and cut fuel to the engines.

FIFY But your original text is likely to be correct in the specific circumstances of this tragedy.

This is a very plausible scenario. Above 400 ‘ AGL, memory items.

Most large aircraft including the 787 have overriding fuel pumps. The centre tank pump on each side delivers the highest pressure so they supply the fuel under normal conditions.

If/when the centre tank is fully used or the pump fails, the two wing tank pumps supply the on-side engine. This happens on every flight that takes off with more than ~34t of fuel (two wing tanks) and lands with less.

If neither on-side pump is operating, the pressure in the supply line drops below that of the tank and pulls open the suction check valve.

The only 'reconfiguration' the pilots can do is open the crossfeed valve, or turn off pumps.